Gas turbines are known to comprise one or more combustion chambers, wherein a fuel is injected, mixed to an air flow and combusted, to generate high pressure flue gases that are expanded in a turbine.
During operation, pressure oscillations may be generated that could cause mechanical damages to the combustion chamber and limit the operating regime.
For this reason, usually combustion chambers are provided with damping devices, such as quarter wave tubes, Helmholtz dampers or acoustic screens, to damp these pressure oscillations.
With reference to FIG. 1, traditional Helmholtz dampers 1 include a damping volume 2 (i.e. a resonator volume) and a neck 3 (an entrance portion) that are connected to a wall 4 (shown by line pattern) of a combustion chamber 5 where the combustion occurs and pressure oscillations to be damped are generated.
The resonance frequency (i.e. the damped frequency) of the Helmholtz damper depends on the geometrical features of the damping volume 2 and entrance portion 3 (neck) and must correspond to the frequency of the pressure oscillations generated in the combustion chamber 5.
Particularly, the volume and neck geometry determine the Eigen frequency of the Helmholtz damper. The maximum damping characteristics of the Helmholtz damper is achieved at the Eigen frequency and it is typically in a very narrow frequency band.
Nevertheless, frequency of these pressure oscillations may slightly change from gas turbine to gas turbine and, in addition, also for the same gas turbine it may slightly change during gas turbine operation (for example part load, base load, transition).
Since at the low frequency range (where Helmholtz dampers are often used) the damping frequency bandwidth of the Helmholtz dampers is very narrow, such frequency shifting of the pressure oscillations generated in a combustion chamber could render a Helmholtz damper connected to it and having a prefixed design resonance frequency completely useless.
In order to address pressure oscillations having different frequencies, typically two or more Helmholtz dampers are used. For example, DE 102005062284 discloses a damper arrangement having two or also more than two Helmholtz dampers connected in series, i.e. the neck of a Helmholtz damper is connected to the volume of another Helmholtz damper. This arrangement proved to be quite efficient in damping pressure oscillation having different, far apart frequencies, such as for example 15 Hz and 90 Hz.
It is clear that in this case some of the pressure oscillations would not be damped, with a detrimental effect on the gas turbine structure and operation.
In order to damp pressure oscillations in a sufficiently large bandwidth, typically a number of Helmholtz dampers 1 are connected to the combustion chamber 5.
Nevertheless, also in this case problems may arise.
In fact, in order to efficiently damp pressure oscillations having a fixed frequency, a Helmholtz damper must be located at the position of the combustion chamber where that pressure oscillations have maximum amplitude.
It is clear that when a combustion chamber has the pressure oscillations with different frequencies having maximum amplitude at the same location or at locations close to one another, different Helmholtz dampers having different features should be installed at those locations.
Nevertheless, in combustion chamber of a gas turbine the locations where Helmholtz dampers can be connected are limited and, thus, it is usually not possible to connect different Helmholtz dampers at the same location (for example angularly shifted from one another).
Moreover, another disadvantage of this multiple Helmholtz damper arrangement is that combustion chamber's pulsations will be attenuated only by those Helmholtz dampers that have resonant frequency sufficiently close to the pulsation frequency.
This concept is explained in detail with reference to some other prior art figures. FIG. 2A shows a wave pattern corresponding to a pulsation frequency FA with amplitude A0 in combustion chamber 5. It will be apparent to a person skilled in the art that the wave pattern represented is a simplified form of a standing wave pattern shown in one timestamp having two anti-nodes, i.e., maximum amplitude points of the waveform. Now, as shown in FIG. 2B, damper 1 (which is pre-tuned to address the frequency FA) is positioned at the peak of this frequency FA curve, thus it results in just damping the wave pattern at that position. In other words, damper 1 is positioned closest to one anti-node of the wave pattern of frequency FA. This eventually results in shifting of mode of frequency FA (i.e., the anti-node shifts position) without actually having any affect on the amplitude A0.
With reference to FIGS. 3A and 3B, two Helmholtz dampers 1 are positioned closest to the anti-nodes of frequency FA, such that it results in decrease of amplitude of frequency wave FA from A0 to A1. However, it may give rise to another mode B at a different frequency FB having amplitude B0. Hence, if one dominant mode is acted upon by using multiple dampers, another mode may become dominant and governs the dynamics of the system. Although, the amplitude of mode A is reduced but the rise of mode B increases the pulsations in combustion chamber 5. Practically, combustion systems with more than one mode are unstable.
To deal with the above-mentioned problem of two modes, two dampers tuned to two different frequencies may be used. As shown in FIGS. 4A and 4B, a second Helmholtz damper 10 tuned to frequency FB is used. However, in this case, the first mode A is again not completely damped but gets shifted as was explained in FIG. 2B. Moreover, the second mode B may not also get completely damped by damper 10, as it may not have an anti-node at the location where damper 10 is connected to combustion chamber 5. Thus, even by using two dampers tuned to two different frequencies FA and FB it does not ensure that both frequencies will be damped at the locations where both dampers contact the combustion chamber.
Hence, all above-mentioned solutions suffer from the issue of not being able to address all relevant frequencies to provide a broadband damping. Moreover, these solutions do not allow fine tuning of the resonance frequency to follow shifting of the frequency pressure oscillations in the combustion chamber.